Lateral self-aligned transistors have been utilized in the semiconductor industry to provide improved bipolar integrated circuits. This type of structure allows for a more densely packed circuit without resorting to a vertically fabricated device utilizing, for example, the conventional buried collector. One disadvantage to the fabrication of lateral bipolar devices, has been the inability to utilize a top side base contact and also the doping profile of the base, collector and emitter of the transistor.
In prior art lateral NPN transistors, a uniform base doping profile has typically been utilized. This is to be compared with the graded base dopant profile for a vertical transistor. The graded base dopant profile has operational advantages over the uniform base in that it allows for an optimized design with enhanced carrier velocity. The uniform base of prior art lateral devices results from the construction method which normally requires the emitter and the collector to be diffused or implanted into a previously formed epitaxial base layer, which has a uniform dopant distribution.
An additional disadvantage with respect to previously fabricated lateral NPN transistors is the use of a buried conductor. Typically, a lateral transistor utilizes a buried P+ layer extending under the transistor to provide the base contact, with an external contact point provided on the periphery of the transistor. In addition to the fabrication problems associated with this type of structure, there is also an associated vertical parasitic transistor. Additionally, the contact is made underneath the transistor to the buried contact as opposed to a top side contact. It is more desirable to have a top side contact.
In view of the above disadvantages, there exists a need for a lateral NPN transistor which has both a graded junction and a top side base contact in addition to increased isolation to prevent the existence of parasitic transistors.